1. Field of the invention
This invention relates to a pulse oximeter with which the oxygen saturation of arterial blood flowing through the body of an object can be measured continuously in a non-invasive manner using the principle that light of two different wavelengths, say, red light and infrared light, have different light absorbent characteristics in the object.
2. Related Art
Pulse oximeters are conventionally used to determine the oxygen saturation of arterial blood in a non-invasive manner. The operating principle of the pulse oximeter is as follows: at least two wavelengths of light (typically, red light and infrared light) that have different transmission characteristics for oxyhemoglobin and deoxyhemoglobin are launched into a fingertip or earlobe, and the light that has been transmitted through or reflected from the living tissue is received, and the arterial oxygen saturation is calculated on the basis of the fact that the light absorbency of blood differs at the respective wavelengths of interest depending upon the oxygen saturation.
The waveforms of the received light contain pulsating components due to variations in blood flow with the pulse rate of the object. The pulsating waveforms that are obtained for two or more wavelengths of light are ideally similar in shape and the only difference is in amplitude on account of the difference in light absorbency.
If the reception waveform (after logarithmic process) that is obtained with the light of one wavelength is written as x(t), the reception waveform (after logarithmic process) that is obtained with the light of the other wavelength is written as y(t), x(t) and y(t) which are similar in shape can be related by: EQU x(t)=K.multidot.y(t)
where K is the proportionality constant. Thus, the oxygen saturation (SpO.sub.2) of arterial blood has ideally one-to-one correspondence to the proportionality constant K,
FIG. 5 is a block diagram of a conventional pulse oximeter that operates on the above-described principle of measurement. As shown, a LED drive circuit 1 is connected to two light-emitting diodes (LED) 2 and 3 that constitute light-emitting devices; typically, one LED 2 is for emitting red light at a wavelength of 660 nm and the other LED 3 is for emitting infrared light at a wavelength of 940 nm. Since the LED drive circuit 1 is under the control of a timing generator circuit 9, the LEDs 2 and 3 emit red and infrared light alternately at a repetition period as shown in FIG. 6, which light are then launched into a living tissue 4 through which arterial blood flows. The two wavelengths of light that are transmitted through or reflected from the living tissue 4 are received by a photodiode 5 working as a light-receiving device and thence enter a current voltage converter circuit (I-V converter circuit) 6 so that they are converted to voltage signals. The waveforms of signal leaving the I-V converter circuit 6 are passed through a bandpass filter (BPF) 7, thence to a demodulator 8. In response to a timing signal from the timing generator circuit 9, the reception waveform of red light is distributed to a logarithmic amplifier (log AMP) 10 whereas the reception waveform of infrared light is sent to another logarithmic amplifier (log AMP) 11.
After logarithmic processing in the amplifiers 10 and 11, the reception waveforms x(t) and y(t) for red and infrared light, respectively, are isolated into a waveform buffer 25, with x(t) being also fed to a peak-bottom detector 26. In the peak-bottom detector 26, both peaks and bottoms of the input waveform x(t) are detected and time-related sequence information of peaks and bottoms is sent to the waveform buffer 25. On the basis of his time-related sequence information, maximum displacements .increment.x and .increment.y per heart beat are determined for the input waveforms x(t) and y(t), respectively, and the signals for .increment.x and .increment.y are sent to a divider 27.
In the divider 27, constant K (=.increment.x/.increment.y) which has one-to-one correspondence to the oxygen saturation is calculated and the thus determined K is sent to the next computing stage 28, where the oxygen saturation S is determined by S=n(K). The thus calculated oxygen saturation S is displayed on a display 29.
The logarithmic amplifiers 10 and 11, waveform buffer 25, peak-bottom detector 26, divider 27 and computing stage 28 can be configured in a microcomputer, so that signals from the demodulator 8 are converted to a digital form by means of an A/D converter before they are fed into the microcomputer for the necessary processing.
As described above, constant K=.increment.x/.increment.y ideally has one-to-one correspondence to the oxygen saturation. In fact, however, the reception waveforms x(t) and y(t) are variable with an unexpected factor due to the influence of such effects as the site and method of sensor attachment. Stated more specifically, the pulsating waveform in each reception waveform should ideally consist of the pulsating component of arterial blood but, in fact, external effects such as the displacement of the attached sensor, body movements and incoming light will introduce irregular noise (artifacts) other than the pulsation component.
If such irregular noise is involved in the process of calculating the oxygen saturation using the amplitude information of reception waveforms x(t) and y(t), the calculated value will naturally have low reliability.
Therefore, for precise measurement of the arterial oxygen saturation using a pulse oximeter, it is critical to provide for positive elimination of the erratic occurrence of irregular noise.